1. Field Of The Invention
The present invention relates generally to imaging systems or camera systems (such as photographic and video cameras), and more particularly to exposure controls for imaging systems/camera systems.
2. Related Art
The use of charge coupled devices (CCDs) in camera imagers has enabled the production of smaller and more durable camera imagers. Much effort has been spent in developing exposure control systems for effectively controlling imager sensitivity to incident light on the imager (that is, for controlling the amount of charge a CCD accumulates during a field integration period). Exposure control systems which regulate CCD exposure sensitivity, such that picture quality is unaffected by changing environmental lighting conditions, are desired. Such exposure control systems greatly expand the dynamic range of performance of CCD-based imagers, thereby increasing the commercial market for CCD-based camera imagers. Herein, light refers to visible and non-visible light.
In many conventional CCD-based camera imagers, the exposure control systems are implemented using mechanical devices. Such mechanical devices include a mechanical iris and/or a mechanical shutter wheel. The most common implementation, the mechanical iris, varies the rate at which the CCD receives photons. The mechanical shutter varies the amount of time during which the CCD receives photons.
Such exposure control systems are flawed, however, since mechanical devices have a relatively low reliability, slow response time, and increase the size and cost of the lens. Additionally, such exposure control systems are flawed since the use of mechanical devices increases the technical difficulty in the areas of loop stability and design transition between different lenses.
In some tube based cameras, such as some "VIDICON" cameras, the exposure control systems can be non-mechanical in nature. Such exposure control systems automatically control imager sensitivity by controlling the voltage on the tube's target electrode.
Since vacuum tube type imagers are larger and less rugged than solid state imagers, solid state imager based cameras such as CCD cameras are being used in place of the older tube technology based cameras. In order to match all performance aspects of tube based cameras with CCD based cameras, a non-mechanical exposure control system for controlling CCD sensitivity is required. Due to the different nature of the solid-state imager to the tube imager, the same electronic exposure control technique as the tube-based imager cannot be utilized for the solid-state imagers. There is no analogous sensitivity altering technique for the solid-state imager similar to the tube's target electrode voltage. Therefore, a different technique to achieve similar electronic light control for the solid state imager is required.
Several major CCD suppliers offer on-chip shuttering options. On-chip shuttering is the electronic process that allows accumulated charge in the charge storage wells to be dumped before the end of the normal integration time. This allows the exposure interval per-imaging cycle to be varied, enabling high speed stop action effects. This change in exposure interval also results in a corresponding change in sensor sensitivity. Currently, light control systems based solely on on-chip shuttering do not exist. This is due to the many restrictions imposed due to CCD timing constraints and undesirable imaging side-effects upon application of an electronically variable exposure setting.
Thus, a high performance exposure control system for effectively controlling CCD sensitivity is required.